The technology described is in the field of fuel cells especially proton exchange membrane (PEM) fuel cells.
This subject matter of the disclosure relates to a component known as the bipolar plate, also known as interconnect plate, which serves as one of the key components employed in the construction of fuel cells.
More specifically, the subject matter of the disclosure relates to protective coatings for metallic, plastic or composite bipolar plates in which the protective coatings function as barriers against corrosion and ion contamination of the proton exchange membrane (PEM).
The schematic diagram of a typical PEM fuel cell using hydrogen as fuel is shown in FIG. 1 in cross-section (FIG. 1A) and exploded (FIG. 1B) views. A PEM fuel cells typically consists of a stack (100) of repeating units or single cells (110), each cell made of individual components. The cell (110) consists of a bipolar flow plate (120) containing an inlet/outlet (I/O) side (120a) for the hydrogen fuel gas and water vapor, a porous anode (130), a polymer electrolyte membrane (140), a porous cathode (150), and an oxygen/air inlet/outlet side (120b) of the flow plate (120). A single plate, in which the I/O flow patterns for the fuel (anodic side) and I/O pattern for oxygen/air inlet/outlet (cathodic side) are engraved or embossed on the opposite sides, is referred to as the “bipolar plate” in a PEM fuel cell.
Fuel hydrogen gas (160) entering the flow paths (165) of bipolar flow plate (120) on the anode side is ionized to protons and electrons on the catalyst surface at the anode. The protons transport through the proton exchange membrane (140) and reach the cathode (150). The electrons leaving the anode travel through an external load to reach the cathode, where they react with the oxygen/air supplied (170) as oxidant through the flow channels (175) on the cathode side of the bipolar plate and form oxide anions. The protons reaching the cathode react with the oxygen ions generated at the cathode to form water.
A schematic diagram of a conventional bipolar flow plate is shown in FIG. 2. The bipolar plate (200) consists of an inlet port for fuel gas and moisture (210) and an outlet port for the unused gas (230). The gas passes through the flow channels (220) engraved or embossed in the plate for directing uniform flow of the gases over the electrode surface for maximal contact. The flow channel paths can be of a variety of types, most common being serpentine path and a parallel flow path. A typical serpentine flow channel path (220) for inlet gases is also shown in FIG. 2A. FIG. 2B is an enlarged view of the flow path. A similar flow channel (not shown) is found on the opposite face of the flow plate for passage of oxidant gases past the cathode. In a fuel cell, the bipolar flow plates are stacked in such a way that the gas inlet channels engraved on both sides for the fuel gas and the oxidant gas respectively contact their respective electrodes.
Due to the complexity of the processes that take place in a PEM fuel cell, a bipolar plate fulfills several important functions in a fuel cell, including:    (a) Distribution of the fuel and oxidant separately and evenly over the respective electrode areas;    (b) Providing mechanical support to the membrane exchange assembly;    (c) Electron transport;    (d) Accommodation of internal manifolds for gases and coolant liquids;    (e) Handling stack seal stresses, and    (f) Thermal management within the cell.
Bipolar flow plates serve as electronic conductors in the anodic as well the cathodic side. This requires them to have excellent through-plate electronic conductivity but no ionic conductivity or gas permeation through them. Some of the material characteristics that are needed to meet the functional requirements of the bipolar plate in a fuel cell are (a) high electrical and thermal conductivity (b) poor hydrogen permeability (c) high mechanical strength (d) low density and (e) easy manufacturability at low cost.
Bipolar plates have been made out of a variety of materials and methods, most often solid blocks of machined graphite. Machined metal plates mostly of stainless steel are also known in the prior art. Another type of flow plate is a filled polymer composite material.
Each of these approaches has certain advantages but also faces major difficulties on one or another important requirement. For example, graphite bipolar plates are considered state of the art, but have huge machining costs and also lack the mechanical strength. Metals, while being excellent conductors and having lower material costs, do not have the corrosion resistance required in an aggressive redox environment. Filled polymer composites do not meet critical through-plane conductivity requirements or gas-permeability requirements.
Among the composite types, compression molded graphite particles in a thermoplastic polymer have been examined in detail. The process involves mixing graphite and thermoplastic granules in a mold, and heating it above the glass transition temperature (Tg) of the polymer under pressure until the materials mix together and flow into the mold. The major advantage claimed by this method is the ability to load higher volume fractions of the filler, thereby increasing the electrical conductivity. However, this method is marked by a slow production cycle limited by the cooling cycle for the mold.
Carbon-carbon composites are also suitable materials for bipolar plates. Simple graphite-carbon composite systems are noted for their advantages of lower contact resistance, high corrosion resistance and easy manufacturability. But, they are limited by poor bulk electric conduction, low volume density for power and gas permeation rates that leave large room for improvement. More complex systems, e.g., a three-component carbon-polymer-metal system, can provide better performance, but the cost of manufacturing is prohibitive.
Metals such as aluminum, titanium, nickel or alloys like stainless steel as materials for bipolar plates have advantages due to their better mechanical properties, higher electrical conductivity, lower gas permeability and low cost of manufacture. However, metals have two serious limitations in terms of the electrochemical processes that take place at their surface: (a) formation of non-conductive surface oxides (corrosives) in a PEM fuel cell environment resulting in a high contact resistance which eventually lowers the efficiency of the PEM fuel cell system and (b) the dissolution of metal cations from the alloys and their subsequent contamination of the membrane electrode assembly (e.g., anode, separator and cathode assembly) will cause eventual system failure.
A recognized method to solve the corrosion problem has been to coat the surface of the metal bipolar plate with a material that forms a barrier to corrosion and at the same time will not diminish the advantageous properties of the metallic bipolar plate. Some of the promising corrosion barrier coatings that have been tested on metal plates including stainless steel plate surfaces include chromium nitride (CrN) and titanium nitride (TiN). However, high vacuum conditions and high temperatures (ca. 900° C). required to ensure the formation of non-brittle phases of CrN needed for this approach limit its scale and therefore the low cost manufacturability of this approach. In addition, the presence of metal ions from the barrier layer leaves the potential for the diffusive contamination through the barrier layer into the membrane electrode assembly.
Carbon nanotubes (CNT) are seamless tubes formed from a single sheet of graphite (graphene). CNTs are well known for their superior electrical, mechanical and thermal properties arising from their unique electronic structure.
Carbon nanotubes have been used to coat metal bipolar plates. The CNTs are deposited by a chemical vapor deposition method mainly to render the bipolar plate hydrophilic for better water management properties in the PEM fuel cell. The growth of defect free layers of carbon nanotubes by chemical vapor deposition on large area metal substrates is prohibitively expensive for practical usage.